This invention relates to a method of forming sandwich structures, comprising one or more non-corrugated sheet elements (liners) to which are bonded on e or two corrugated (fluted) sheet elements (mediums) which are themselves bonded together.
U.S. Pat. No. 4,886,563 discloses an improved corrugated paper board structure in which two layers of corrugated medium (flutes) are bonded together at the peaks of the flutes, and are sandwiched between two outer liners. The disclosures of U.S. Pat. No. 4,886,563 are incorporated herein by reference.
U.S. Pat. No. 4,886,563 also discloses a method of forming the structural paper in which two separate layers of corrugating medium are corrugated and then brought into flute tip to flute tip contact on synchronised corrugating rollers.
U.S. Pat. No. 3,700,518 disclosed a method of forming board having two outer liners and two inner fluted mediums bonded at their flute peaks. The method adopted is to corrugate two mediums together, bond liners to each separate the two single faced boards and adhere the boards together at the flute peaks of the fluted mediums. This method has the disadvantage of not being able to accurately bond the flute peaks together with pressure.
Australian Pat. No. Application 11926/88 addressed the problem of capital cost in constructing a new corrugating plant for the flute tip to flute tip paper board. As such, specification 11962/88 focused on a modification to existing corrugating machinery which enabled conventional corrugated board, as well as the new flute tip to flute tip corrugated board to be made.
Conventional machinery for forming corrugated board incorporates a unit for making single faced corrugated board, (a liner adhered to a single fluted layer). More complex board constructions can be formed by bonding the exposed flutes to another liner and, if desired, subsequently bonding that to another single faced corrugated board.
A "single facer" plant normally includes as its essential components:
(a) Mill roll stands for liner and corrugating medium. PA1 (b) A pair of corrugating rollers for corrugating the medium into a fluted medium. PA1 (c) A pair of heated smooth (non corrugated) guide rolls for the liner board. PA1 (d) A non corrugated pressure roll for bringing the liner into contact with the second corrugating roller to form the single faced board. PA1 (e) An idler roll to guide the formed single faced board to subsequent processing units. PA1 (a) mill roll stands for two mediums and one liner; PA1 (b) a carrier roll fluted to support a first fluted medium; PA1 (c) a single corrugating roll of diameter no larger than the carrier roll engaging with the carrier roll to corrugate the first fluted medium; PA1 (d) a pair of corrugating rolls of diameter no larger than the carrier roll adapted to produce a second fluted medium said pair being located and driven relative to the carrier roll to enable the first and second fluted mediums to be brought into flute tip to flute tip contact and to be conveyed by the carrier roll, the axes of the corrugating rolls not lying in a plane common with the single corrugating roll or the carrier roll; PA1 (e) a first adhesive application station to apply adhesive to one of said fluted mediums prior to the flute tips of the said first and second fluted mediums coming into contact PA1 (f) a second adhesive application station to apply adhesive to the exposed flute tips of the bonded mediums or to a liner board; PA1 (g) a liner applicator roll located adjacent to said carrier roll downstream of said second adhesive applicator positioned to lead a liner board into contact with the bonded fluted mediums without application of pressure; PA1 (h) an energy radiator unit located adjacent said carrier roll downstream of said liner applicator to accelerate curing of the adhesive bond between the liner and the mediums; PA1 (i) the diameter of said carrier roll and the positioning of said pair of corrugating rolls being arranged to ensure there is sufficient remaining circumference on said carrier roll to enable the fluted mediums to be retained until after they have passed the curing zone of the said energy radiator.
With conventional single faced board a degree of pressure is applied to ensure secure bonding between the liner and the corrugated medium. This application of pressure creates pressure lines on the outer face of the liner corresponding to the position of the flute tips. The need to apply pressure can adversely affect the strength of the single faced structure and adversely affects the appearance of the board and its suitability for printing.